Carbon Fiber Strand for Reinforcing Thermoplastic Resins and Method of Producing the Same

ABSTRACT

There are disclosed a carbon fiber strand for reinforcement of thermoplastic resin, wherein a sizing agent containing a resin composition obtained by mixing a component [A], i.e. an acid-modified polyolefin copolymer having a weight-average molecular weight of 15,000 to 150,000, which has, as the main chain, an ethylene-propylene copolymer, a propylene-butene copolymer or an ethylene-propylene-butene copolymer and in which the main chain has been modified with 0.1 to 20% by mass of an unsaturated carboxylic acid, and a component [B], i.e. an acid-modified polypropylene having a weight-average molecular weight of 3,000 to 150,000, which has, as the main chain, a polypropylene and in which the main chain has been modified with 0.1 to 20% by mass of an unsaturated carboxylic acid, at a mass ratio of 1:20 to 10:5, is adhered to 100 parts by mass of a carbon fiber in an amount of 0.1 to 8.0 parts by mass; a method for producing the above strand; and a carbon fiber-reinforced thermoplastic resin containing the above strand.

TECHNICAL FIELD

The present invention relates to a carbon fiber strand for reinforcementof thermoplastic resin, having excellent adhesivity to thermoplasticresins such as polypropylene and the like; a method for productionthereof; and a carbon fiber-reinforced thermoplastic resin reinforcedwith the strand.

BACKGROUND ART

Carbon fiber and carbon fiber-reinforced thermoplastic resins producedusing the carbon fiber as a reinforcing material (the resins mayhereinafter be referred to as composite material) have various superiorproperties such as high tensile strength and high tensile modulus,excellent heat resistance, excellent chemical resistance, excellentfatigue characteristic and excellent abrasion resistance, small linearexpansion coefficient and consequent excellent dimensional stability,excellent electromagnetic wave-shieldability, high X-raytransmittability and the like. Therefore, they are in wide use inapplications where the above properties are required, such as sports,leisure, aerospace industry, general industries and the like.

Conventional, carbon fiber-reinforced thermoplastic resins are producedin many cases, using a thermosetting resin such as epoxy resin or thelike as the matrix. Recently, however, attention has been paid to athermoplastic resin as a matrix resin, from the standpoint ofrecyclability and rapid moldability.

Carbon fiber-reinforced thermoplastic resins using a thermoplastic resinas the matrix are produced by a molding method such as injection moldingof compound pellets, injection molding of long fiber pellets,injection-compression molding, extrusion, stamping using a random mat,or the like. In the carbon fiber-reinforced thermoplastic resinsproduced by such a molding method, the length of carbon fiber isrelatively short. Consequently, the mechanical properties (e.g. strengthand modulus) of the carbon fiber-reinforced thermoplastic resinsproduced are greatly influenced by the affinity and adhesivity betweenthe carbon fiber and the thermoplastic resin (matrix).

As the matrix resin used in the carbon fiber-reinforced thermoplasticresin, there can be mentioned, for example,acrylonitrile-butadiene-styrene copolymer (ABS), polyamide (e.g. nylon 6or nylon 66), polyacetal, polycarbonate, polypropylene, high-densitypolyethylene, polyethylene terephthalate, polybutylene terephthalate,polyetherimide, polystyrene, polyethersulfone, polyphenylene sulfide,polyetherketone and polyetheretherketone.

Of these thermoplastic resins, polypropylene resin is inexpensive andhaving superior qualities in moldability, water resistance, chemicalresistance (oil resistance and solvent resistance), electricalinsulation, etc. Therefore, use of polypropylene resin as a matrix ofcarbon fiber-reinforced thermoplastic resin is expected to increasestrikingly in the future. However, since polypropylene resin is acrystalline resin and moreover has no polar group in the molecule, ithas low affinity with carbon fiber. For this reason, conventional,carbon fiber-reinforced thermoplastic resins obtained by reinforcingpolypropylene resin with carbon fiber are relatively low in mechanicalproperties.

Carbon fiber strand is constituted by a large number of ultrafine carbonfibers. Such carbon fiber strand has a small elongation and, whensubjected to mechanical friction, etc., tends to generate fluff. Tocarbon fiber strand is ordinarily added a sizing agent in order toprevent the generation of fluff and improve the collectability of carbonfiber to improve the handleability of carbon fiber and, in producing acarbon fiber-reinforced thermoplastic resin, to improve the affinity ofcarbon fiber strand with thermoplastic resin (matrix).

As the sizing agent for carbon fiber, many proposals have been madeheretofore. For example, in JP-A-2005-48344 is proposed a sizing agentobtained by dispersing, in water, a polyproylene type resin modifiedwith 1 to 20% by mass of an unsaturated dicarboxylic acid and having anintrinsic viscosity of 0.02 to 1.3 dl/g, or a salt thereof.

In JP-A-1990-84566 is proposed a sizing agent composed of anethylene/propylene copolymer modified with a carboxyl group and/or aminogroup-introduced, unsaturated dicarboxylic acid, a polypropylenemodified with the above acid and a polyethylene modified with the aboveacid.

However, the acid-modified polypropylene resin, etc. used in thesesizing agents, however, are each a solid at normal temperature andhighly crystalline. Therefore, the sizing agents have a low effect forprevention of fluffing of carbon fiber strand. Further, a carbon fiberstrand to which such a sizing agent has been added, is too high in drapeand difficult to wind solidly in a bobbin form to obtain a productpackage. As a result, there tend to occur such problems as, intransportation of the product package, the carbon fiber strand ofpackage causes slough-off.

For these reasons, it is desired to develop a sizing agent for carbonfiber strand, which has good affinity with thermoplastic resins (e.g.polypropylene resin), is low in fluffing particularly when subjected tofretting, and has appropriate drape.

DISCLOSURE OF THE INVENTION

The present invention has been made with attention paid to theabove-mentioned problems of prior art. The present invention aims atproviding, at low costs, a carbon fiber strand for reinforcement ofthermoplastic resin, which has high adhesivity to thermoplastic resin asa matrix, has excellent collectability and excellent frettingresistance, and has a high effect for reinforcement of thermoplasticresin, and a thermoplastic resin reinforced with the carbon fiberstrand.

The present inventors made a study. As a result, it was found that acarbon fiber strand to which a sizing agent containing a resincomposition obtained by mixing, at a given ratio, a modified polyolefincopolymer and a modified polypropylene both having given structures, hashigh affinity to thermoplastic resins (e.g. polypropylene), hasexcellent collectability and excellent fretting resistance, and can bepreferably used as a reinforcing agent for thermoplastic resin. Thefinding has led to the completion of the present invention.

The present invention which has achieved the above aim, is as describedbelow.

[1] A carbon fiber strand for reinforcement of thermoplastic resin,wherein a sizing agent containing a resin composition obtained by mixingthe following components [A] and [B]:

[A] an acid-modified polyolefin copolymer having a weight-averagemolecular weight of 15,000 to 150,000, which has, as the main chain, atleast one member selected from an ethylene-propylene copolymer, apropylene-butene copolymer and an ethylene-propylene-butene copolymerand in which the main chain has been modified with 0.1 to 20% by mass ofan unsaturated carboxylic acid, and

-   [B] an acid-modified polypropylene having a weight-average molecular    weight of 3,000 to 150,000, which has, as the main chain, a    polypropylene and in which the main chain has been modified with 0.1    to 20% by mass of an unsaturated carboxylic acid,    at a mass ratio of 1:20 to 10:5, is adhered to 100 parts by mass of    a carbon fiber in an amount of 0.1 to 8.0 parts by mass.    [2] A carbon fiber-reinforced thermoplastic resin obtained by    adding, to a thermoplastic resin, a carbon fiber strand for    reinforcement of thermoplastic resin, set forth in [1], in an amount    of 5 to 70% by mass.    [3] A carbon fiber-reinforced thermoplastic resin according to [2],    wherein the thermoplastic resin is a polypropylene.    [4] A method for producing a carbon fiber strand for reinforcement    of thermoplastic resin, which comprises immersing a carbon fiber    strand in an aqueous suspension containing a resin composition set    forth in [1] and then heating the resulting carbon fiber strand at a    temperature at least equal to the melting point of the resin    composition.    [5] A method for producing a carbon fiber strand for reinforcement    of thermoplastic resin, which comprises immersing a carbon fiber    strand in either one of an aqueous suspension of [A] an    acid-modified polyolefin copolymer and an aqueous suspension of [B]    an acid-modified polypropylene, drying the resulting carbon fiber    strand, immersing the dried carbon fiber strand in the other aqueous    suspension, and then heating the carbon fiber strand after immersion    in the two aqueous suspensions, at a temperature at least equal to    the melting point of the resin composition.    [6] A method for producing a carbon fiber strand for reinforcement    of thermoplastic resin, which comprises immersing a carbon fiber    strand in an aqueous suspension of [B] an acid-modified    polypropylene, drying the resulting carbon fiber strand, heating the    dried carbon fiber strand at a temperature at least equal to the    melting point of the acid-modified polypropylene, immersing the    resulting carbon fiber strand in an aqueous suspension of [A] an    acid-modified polyolefin copolymer, drying the resulting carbon    fiber strand, and then heating the dried carbon fiber strand at a    temperature which is at least equal to the melting point of the    acid-modified polyolefin copolymer but lower than the melting point    of the acid-modified polypropylene [B].    [7] A method for producing a carbon fiber strand for reinforcement    of thermoplastic resin, set forth in any one of [4] to [6], wherein    the acid-modified polypropylene [B] has the following properties:    (1) a crystallinity of 50 to 80% as measured by IR spectrometry,    (2) an intrinsic viscosity [η] of 0.05 to 0.7 dl/g,    (3) an acid value of 40 to 100 mg-KOH/g, and    (4) a mass reduction of less than 5% when subjected, by a    thermobalance, to temperature elevation from 23° C. to 250° C. at a    rate of 10° C./min in the air.

The carbon fiber strand of the present invention is superior inadhesivity to and affinity with thermoplastic resins such aspolyproylene resin and the like and, when subjected to mechanicalfriction, causes no fluffing at the surface. Further, since havingappropriate drape, the carbon fiber strand of the present invention canbe wound solidly in a bobbin form and made into a product package.

The carbon fiber strand can be preferably used as a reinforcing materialfor thermoplastic resin. A carbon fiber-reinforced thermoplastic resinobtained by adding the carbon fiber strand of the present invention to athermoplastic resin has a strikingly high mechanical strength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing explaining a method for measuring the amount offluff of carbon fiber strand generated when a carbon fiber strand issubjected to fretting by a stainless steel round rod.

FIG. 2 is a drawing explaining an apparatus used for measurement ofdrape.

FIG. 3 is a schematic drawing showing a method for examining theadhesion strength between a carbon fiber filament and a polypropyleneresin.

1 is a carbon fiber strand; 3 is a stainless steel round rod; 20 is asample; 22 is a slit; 24 is a sample stand; 26 is a metal plate; 31 is acarbon fiber filament; 32 is an adhesive; 33 is a backing paper; 33 aand 33 b are each a projected portion; 34 is a micro-droplet; and 35 aand 35 b are each a blade.

BEST MODE FOR CARRYING OUT THE INVENTION

The carbon fiber strand of the present invention is obtained byadhering, to a carbon fiber bundle, a sizing agent containing a resincomposition obtained by mixing, at a given ratio, a component [A], i.e.an acid-modified polyolefin copolymer and a component [B], i.e. anacid-modified polypropylene resin.

The component [A] is a modified polyolefin copolymer which has, as themain chain, at least one member selected from an ethylene-propylenecopolymer, a propylene-butene copolymer and an ethylene-propylene-butenecopolymer and in which the main chain has been graft-modified with 0.1to 20% by mass, based on the total amount of the component [A], of anunsaturated carboxylic acid.

In the main chain of the component [A], the proportion of the propyleneconstituent unit is preferably 50 to 98 mol %. When the proportion ofthe propylene constituent unit is less than 50 mol %, the adhesivitybetween carbon fiber strand and thermoplastic resin matrix tends to below. When the proportion of the propylene constituent unit is more than98 mol %, the carbon fiber strand per se tends to be low in flexibilityand fluffing preventability.

When the main chain of the component [A] is composed of anethylene-propylene-butene copolymer, the proportion of the buteneconstituent unit in the main chain is preferably 0 to 10 mol %.

As the unsaturated carboxylic acid used for graft-modification of themain chain of the component [A], there is preferred an unsaturatedcarboxylic acid having 3 to 8 carbon atoms. As such an unsaturatedcarboxylic acid, there can be used unsaturated monocarboxylic acids,unsaturated dicarboxylic acids, and their derivatives such as esters,acid anhydrides and the like. Specifically, there are preferred maleicacid, maleic anhydride, fumaric acid, itaconic acid, acrylic acid,methacrylic acid, methyl methacrylate, etc. Of these, particularlypreferred are maleic acid, maleic anhydride, acrylic acid, methacrylicacid and methyl methacrylate.

The amount of unsaturated carboxylic acid grafted to main chain is 0.1to 20% by mass based on the total amount of the component [A],preferably 1 to 15% by mass, more preferably 2 to 10% by mass. When theamount of graft is less than 0.1% by mass, the adhesivity between carbonfiber strand and thermoplastic resin matrix is low. When the amount ofgraft is more than 20% by mass, the amount of the propylene constituentunit in the modified polyolefin copolymer [A] is relatively small and,therefore, the adhesivity between carbon fiber strand and polypropylenematrix resin is low. Further, under ordinary reaction conditions,graft-modification with an unsaturated carboxylic acid of an amount ofmore than 20% by mass is difficult.

As the method for graft-modification of main chain, there are thefollowing methods, for example.

Solution method: this is a method wherein a copolymer to become a mainchain is dissolved in an organic solvent (e.g. toluene or xylene), anunsaturated carboxylic acid and an organic peroxide are added to thesolution, and the mixture is heated to give rise to graftpolymerization.

Melting method: this is a method wherein a copolymer to become a mainchain is melted in an autoclave, a kneader and extruder or the like, anunsaturated carboxylic acid and an organic peroxide are added thereto,and the mixture is heated at 120 to 300° C.

The above modification methods are known (JP-A-1990-84566,JP-A-1991-181528).

When the component [A] has carboxyl group derived from an unsaturatedcarboxylic acid, the carboxyl group may have been neutralized asnecessary, in a given proportion. As the basic compound used for theneutralization of carboxyl group, there can be mentioned, for example,alkali metal salts such as sodium hydroxide, potassium hydroxide and thelike; alkaline earth metal salts; ammonia; and amines such asmonomethylamine, dimethylamine, trimethylamine, triethylamine,diisopropylamine, monoethanolamine, diethanolamine,dimethylethanolamine, morpholine and the like.

The weight-average molecular weight of the component [A] is 15,000 to150,000, preferably 30,000 to 80,000. When the weight-average molecularweight is less than 15,000, the adhesivity between carbon fiber andthermoplastic resin is insufficient and it is impossible to obtain athermoplastic resin sufficiently reinforced with a carbon fiber.Further, the carbon fiber strand per se has insufficient collectabilityand, in using such a carbon fiber strand for production of carbonfiber-reinforced thermoplastic resin, etc., the workability is inferior.

When the weight-average molecular weight of the component [A] is morethan 150,000, it is difficult to suspend the component [A] in water; asa result, production of an aqueous sizing agent (a suspension) isdifficult. Even if a suspension is obtainable, the particles in thesuspension have large diameters, making them unstable and allowing thesizing agent to be unsuitable for long-term operation.

The analysis of the modified polyolefin or modified propylene containedin the sizing agent can be conducted by known methods, for example, amethod of conducting molecular weight fractionation by columnfractionation, dissolution fraction or the like and subjecting eachfraction obtained to compositional analysis by IR or NMR, and a methodof conducting compositional fractionation by crystallizationfractionation and subjecting each fraction obtained to GPC (gelpermeation chromatography) to determine molecular weights. There canalso be used high temperature GPC-FTIR, TREF (temperature rising elutionfractionation)-FTIR, CFC (cross fractionation chromatography)-FTIR, etc.

The ethylene-propylene copolymer, propylene-butene copolymer andethylene-propylene-butene copolymer constituting the main chain of thecomponent [A] are amorphous or low-crystalline polyolefin copolymers.Therefore, when the component [A] is adhered onto the surface of carbonfiber, the resulting film of sizing agent is smooth and flexible.Further, the sizing agent containing the component [A] allows the sizingagent-added carbon fiber strand to have high collectability. Thus, thesizing agent containing the component [A] contributes preferably toprevention of fluffing of carbon fiber strand, prevention of breakage ofcarbon fiber, and control of drape of carbon fiber strand.

The component [B], i.e. the acid-modified polypropylene resin is a resinwhich has a polypropylene as the main chain and in which the main chainhas been graft-modified with an unsaturated carboxylic acid.

The component [B] is an acid-modified polyproylene resin obtained bygraft-modifying a polypropylene as a main chain with 0.1 to 20% by mass,based on the total amount of the component [B], of an unsaturatedcarboxylic acid.

The unsaturated carboxylic acid used in the graft-modification of themain chain is the same as the unsaturated carboxylic acid used ingraft-modification of the component [A]. The preferred amount ofunsaturated carboxylic acid grafted to main chain and the method forgraft-modification are also the same as in the component [A].

The component [B] has carboxyl group derived from an unsaturatedcarboxylic acid, and this carboxyl group may have been neutralized asnecessary, in a given proportion. As the basic compound used forneutralization of the carboxyl group, the same compounds as in thecomponent [A] can be used.

The weight-average molecular weight of the component [B] (modifiedpolypropylene resin) is 3,000 to 150,000, preferably 30,000 to 70,000.When the weight-average molecular weight is less than 3,000, theadhesivity between carbon fiber and thermoplastic resin is insufficientand it is impossible to obtain a thermoplastic resin sufficientlyreinforced with a carbon fiber. Further, the carbon fiber strand per sehas insufficient collectability and, in using such a carbon fiber strandfor production of carbon fiber-reinforced thermoplastic resin, etc., theworkability is inferior.

When the weight-average molecular weight of the component [B] is morethan 150,000, it is difficult to suspend the component [B] in water; asa result, production of an aqueous sizing agent (a suspension) isdifficult. Even if a suspension is obtainable, the particles in thesuspension have large diameters, making them unstable and allowing thesizing agent to be unsuitable for long-term operation.

The component [B], i.e. the acid-modified polypropylene resin ispreferred to have the following properties (1) to (4).

(1) A crystallinity of 50 to 80% as Measured by IR Spectrometry

The acid-modified polypropylene resin is preferred to have acrystallinity of 50 to 80%, preferably 60 to 75% as measured by IRspectrometry. A modified polypropylene resin having a crystallinity ofless than 50%, when used as a sizing agent, has low affinity with athermoplastic resin used as a matrix and the carbon fiber-reinforcedthermoplastic resin obtained has an insufficient strength. Meanwhile,when there is used a modified polypropylene resin having a crystallinityof more than 80%, the sizing agent layer derived from the modifiedpolypropylene resin, which is formed on the surface of carbon fiber, isfragile. As a result, the sizing agent shows a low effect for preventionof fluffing of carbon fiber strand.

The measurement of crystallinity by IR is conducted by calculating aratio of absorbances at 998 cm⁻¹ and 973 cm⁻¹ which are each called acrystal band.

(2) An Intrinsic Viscosity [η] of 0.05 to 0.7 dl/g

The intrinsic viscosity [η] of the modified polypropylene resin ispreferably 0.05 to 0.7 dl/g, more preferably 0.2 to 0.5 dl/g. When acarbon fiber strand containing a modified polypropylene resin having anintrinsic viscosity [η] of less than 0.05 dl/g is used for production ofa carbon fiber-reinforced thermoplastic resin, the amount ofdecomposition gas generated during high-temperature molding is large. Asa result, the adhesivity between carbon fiber and thermoplastic resinmatrix is impaired and the environmental condition of molding operationbecomes bad. Meanwhile, when the intrinsic viscosity of the modifiedpolypropylene resin is more than 0.7 dl/g, the resin's suspension inwater is difficult and, even if the suspension is possible, thesuspension obtained is unstable.

Incidentally, the intrinsic viscosity [η] is a value measured at 135° C.in decalin based on JIS K 7367-3.

(3) An Acid Value of 40 to 100 mg-KOH/g

The acid value of the modified polypropylene resin is preferably 40 to100 mg-KOH/g, more preferably 50 to 80 mg-KOH/g. A carbon fiber strandproduced using an acid-modified polypropylene resin having an acid valueof less than 40 mg-KOH/g, when kneaded with a matrix resin, showsinsufficient adhesivity to the matrix resin. Meanwhile, in the case ofan acid-modified polypropylene resin having an acid value of more than100 mg-KOH/g, the content of polypropylene unit in polypropylene mainchain is relatively low and, as a result, the crystallinity ofacid-modified polypropylene resin is low. Consequently, in kneading acarbon fiber strand produced using such an acid-modified polypropyleneresin, with a matrix resin, the adhesivity between matrix resin andcarbon fiber strand is insufficient.

The acid value is a value measured based on JIS K 0070. That is, theacid value is determined by dissolving a modified polypropylene resin intoluene and conducting titration with an ethanol solution of potassiumhydroxide in the presence of phenolphthalein.

(4) A Mass Reduction of Less than 5% when Subjected to TemperatureElevation in the Air from 23° C. to 250° C. at 10° C./min

The acid-modified polypropylene resin is preferred to show a massreduction of less than 5%, preferably 3% or less when subjected totemperature elevation from 23° C. to 250° C. at 10° C./min in an airenvironment and measured using a thermobalance. When the mass reductionis 5% or more and when there is used a carbon fiber strand containingsuch an acid-modified propylene resin for production of a carbonfiber-reinforced thermoplastic resin, a decomposition gas is generatedin a large amount during high-temperature molding and the adhesivitybetween carbon fiber and thermoplastic resin matrix is impaired.Further, the decomposition gas deteriorates the working environment.

In the carbon fiber strand of the present invention, there is adhered,to a carbon fiber, a sizing agent containing a resin compositionobtained by mixing the above-mentioned components [A] and [B] at a massratio of 1:20 to 10:5, preferably 1:10 to 10:5, more preferably 1:10 to10:10.

When the proportion of the component [A] is smaller than the aboverange, the resulting carbon fiber strand tends to cause fluffing easilyand its handleability is low. When the proportion of the component [B]is smaller than the above range, the resulting carbon fiber strand haslow adhesivity to a thermoplastic resin matrix.

The sizing agent used in the carbon fiber strand of the presentinvention may contain various additives besides the above-mentionedresin composition. As the additives, there are mentioned, for example,synthetic lubricants such as methyl oleate, dioctyl sebacate and thelike; vegetable oils; higher alcohols such as sperm alcohol and thelike; and emulsifiers such as polyethylene glycol type nonionicsurfactant, sulfonated oil of low sulfonation degree, and the like. Thecontent of these additives is preferably 30% by mass or less,particularly preferably 20% by mass or less based on the sizing agent.

The amount of the resin composition composed of the components [A] and[B], adhered to a carbon fiber strand differs depending upon the moldingmethod of a carbon fiber-reinforced thermoplastic resin to be produced,the application thereof, etc. Generally, the amount is preferably 1 to5.0 parts by mass, more preferably 1 to 3 parts by mass relative to 100parts by mass of the carbon fiber strand (containing no sizing agent).When the amount of the resin composition adhered is less than 1 part bymass, the handleability of carbon fiber strand during molding tends tobe inferior. Meanwhile, when the amount is more than 5.0 parts by mass,the drape of carbon fiber strand is low, the winding density of carbonfiber strand when wound round a bobbin is low, and there may occurslough-off, etc. during the transportation of carbon fiber strand.

In adhering, to a carbon fiber strand, a resin composition comprisingthe components [A] and [B] or a sizing agent containing the resincomposition, it is preferred to adhere to the carbon fiber strand, theresin composition or the sizing agent in a suspension form wherein theresin composition or the sizing agent is dispersed in water.

An example of the method for producing the carbon fiber strand of thepresent invention is explained below.

Carbon Fiber as Raw Material

The carbon fiber as a raw material for production of the carbon fiberstrand of the present invention can be any carbon fiber ofpolyacrylonitrile (PAN) type, petroleum or coal pitch type, rayon type,lignin type, etc. PAN type carbon fiber made from PAN is particularlypreferred because it is superior in industrial scale productivity andmechanical properties.

PAN type carbon fiber is a filament of about 6 to 8 μm in diameter andis available in the market as a bundle of about 1,000 to 50,000 suchfilaments.

Carbon fiber strand is produced via about the following four steps.

In the oxidation step, an acrylic fiber is heated in an air environmentof 200 to 300° C. for ring closure of nitrile group and introduction ofoxygen into the acrylic fiber, whereby an infusibilization treatment isconducted for adding high-temperature stability.

In the carbonization step, firing is conducted at high temperatures of1,000° C. or more in an inert gas atmosphere, whereby a carbon fiberhaving a carbon content of 90% by mass or more is obtained.

In the surface treatment step, an oxygen-containing group is introducedonto the surface of the carbon fiber obtained above, whereby theresulting carbon fiber has higher adhesivity to matrix resin. As themethod for surface treatment of carbon fiber, there can be mentioned,for example, chemical solution oxidation or electrolytic oxidation inliquid phase and oxidation in gas phase. Of these surface treatments,electrolytic oxidation in liquid phase is preferred from the stand pointof productivity and uniform treatment. As the electrolyte ofelectrolytic solution used in electrolytic oxidation, there can bementioned, for example, inorganic acids such as sulfuric acid, nitricacid, hydrochloric acid and the like; inorganic hydroxides such assodium hydroxide, potassium hydroxide and the like; and inorganic saltssuch as ammonium sulfate, sodium carbonate, sodium hydrogencarbonate andthe like.

As an index indicating the extent of surface treatment, used inconducting the surface treatment of carbon fiber, there is a surfaceoxygen concentration ratio (O/C) of carbon fiber. The (O/C) can bemeasured by electron spectroscopy for chemical analysis (ESCA). It ispreferred that the electrolytic oxidation is conducted so as to achievean (O/C) of 0.05 to 0.4.

In the sizing step, a sizing agent is adhered to a carbon fiber strandin order to improve the handleability of carbon fiber strand and enhancethe affinity between carbon fiber and matrix resin. In adhering a sizingagent to a carbon fiber, a sizing solution is prepared first.

The sizing solution is an aqueous suspension wherein the component [A]and the component [B] are dispersed in water, and can be produced by,for example, the following method.

The components [A] and [B] are melted with heating and stirring. To themelt is added a basic substance to neutralize unsaturated carboxylicacid-derived carboxyl group and impart ionicity to the resincomposition. A surfactant is added to the mixture, followed by stirringuntil the system becomes uniform. Then, water is added in smallportions, whereby an aqueous suspension is obtained by phase reversal.

As the method for adhering a sizing agent to a carbon fiber strandcontaining no sizing agent, there are a spraying method, a rollerimmersion method, a roller transfer method, etc. Of these adhesionmethods, the roller immersion method is preferred because it is superiorin productivity and uniformity. In this method, a carbon fiber strand ispassed through immersion rollers provided in a sizing agent; during thepassage, opening and squeezing of carbon fiber strand are repeated owingto the action of immersion rollers. By this method, the sizing agent isuniformly impregnated even into the inside of strand.

After the sizing agent has been impregnated between carbon fibers, adrying treatment is conducted for water removal, whereby an intended,sizing agent-adhered carbon fiber strand is obtained. Control of theadhesion amount of sizing agent to carbon fiber is conducted, forexample, by control of concentration of sizing agent or control ofsqueezing of squeezing rollers. Thereafter, the carbon fiber is driedusing, for example, hot air, a hot plate, rollers or an infrared heater.

The dried carbon fiber strand is heated until it reaches a temperatureat least equal to the melting point of the resin composition adhered tocarbon fiber, whereby a thin film of the resin composition adhered tocarbon fiber is formed uniformly on the surface of carbon fiber. Thetime of heating at the temperature at least equal to the melting popintis preferably at least 5 seconds from the moment when the carbon fiberhas reached an intended temperature, and less than 5 minutes issufficient ordinarily.

In the above method for adhesion of sizing agent, a sizing agentcontaining the component [A] and the component [B] was used, and thecomponent [A] and the component [B] were adhered simultaneously to thecarbon fiber. The adhesion method is not restricted to the above methodand the following adhesion method is preferred as well.

In this method, two aqueous suspensions were prepared separately, i.e.an aqueous suspension of [A] an acid-modified polyolefin copolymer andan aqueous suspension of [B] an acid-modified polypropylene are adheredto a carbon fiber in order. In this method, first, a carbon fiber strandcontaining no sizing agent is immersed in either one of the aqueoussuspension of [A] an acid-modified polyolefin copolymer and the aqueoussuspension of [B] an acid-modified polypropylene, followed by drying.Then, the dried carbon fiber is immersed in the other aqueoussuspension. Thereafter, the carbon fiber strand after immersion in thetwo aqueous suspensions is heated at a temperature at least equal to themelting points of the acid-modified polyolefin copolymer and theacid-modified polypropylene. The time of heating is the same asmentioned above. By this heating operation a carbon fiber strand forreinforcement of thermoplastic resin is obtained, wherein a resincomposition (which is a uniform mixture of the components [A] and [B])is coated on the surface of carbon fiber.

The following method is also preferred for production of carbon fiberstrand for reinforcement of thermoplastic resin. In this method, first,a carbon fiber strand containing no sizing agent is immersed in anaqueous suspension of the component [B], i.e. an acid-modifiedpolypropylene and then is dried. Thereafter, the resulting carbon fiberstrand is heated at a temperature at least equal to the melting point ofthe acid-modified polypropylene. The time of heating is the same asmentioned above. As a result, the surface of carbon fiber is covereduniformly with a film of the component [B] (acid-modifiedpolypropylene). The carbon fiber which has been surface-coated by theabove operation, is then immersed in an aqueous suspension of thecomponent [A] (acid-modified polyolefin copolymer), followed by drying.Thereafter, the resulting carbon fiber is heated at a temperature whichis lower than the melting point of the component [B] (acid-modifiedpolyolefin copolymer) but is at least equal to the melting point of thecomponent [A]. The time of heating is the same as mentioned above. Themelting point of the component [B] is higher than the melting point ofthe component [A]. As a result, there is produced a carbon fiber strandfor reinforcement of thermoplastic resin having, on the surface, aninner film of the component [B] and an outer film of the component [A].

The film of the component [A] is very flexible and protects carbon fiberfrom damage by fretting. The film of the component [B] is slightlyfragile but has strikingly high affinity to thermoplastic resin(matrix). As a result, the above-produced carbon fiber strand havingdouble films each composed of a sizing agent, when used forreinforcement of a thermoplastic resin, reinforces the thermoplasticresin highly.

The carbon fiber strand of the present invention is suitable as areinforcing fiber for thermoplastic resin. As the thermoplastic resin,there can be mentioned, for example, acrylonitrile-butadiene-styrenecopolymer (ABS), polyamides (e.g. nylon 6 and nylon 66), polyacetal,polycarbonate, polypropylene, high-density polyethylene, polyethyleneterephthalate, polybutylene terephthalate, polyether imide, polystyrene,polyether sulfone, polyphenylene sulfide, polyetherketone andpolyetheretherketone. Polypropylene is particularly preferred.

The amount of the carbon fiber strand used for reinforcement ofthermoplastic resin differs depending upon, for example, the form ofcarbon fiber and the molding method and application of carbonfiber-reinforced thermoplastic resin. However, from the standpoint ofcost-performance, the amount is preferably 5 to 70% by mass, morepreferably 20 to 40% by mass based on the carbon fiber-reinforcedthermoplastic resin.

In producing a carbon fiber-reinforced thermoplastic resin (a compositematerial) using the carbon fiber strand of the present invention, it ispreferred that, first, the present carbon fiber strand and athermoplastic resin to be reinforced therewith are processed into amolding material such as short-fiber compound, long-fiber pellets,random mat, unidirectionally reinforced prepreg or the like. Then, themolding material can be molded by a method such as extrusion, pressingor the like.

EXAMPLES

Sizing agent-adhered carbon fiber strands were produced under theconditions shown in the following Examples and Comparative Examples. Theproperties of these carbon fiber strands containing a sizing agent weremeasured by the above-mentioned methods or the following methods.

[Method for Measurement of Amount of Adhered Resin Composition]

(1) About 50 g of a sizing agent-adhered carbon fiber strand was takenand its weight (W1) was measured.(2) The carbon fiber strand was washed in pure water to remove theemulsifier contained therein.(3) The resulting carbon fiber strand and toluene (300 ml) were placedin an Erlenmeyer flask with ground stopper and a water-cooling typecooling tube was connected to the flask. The Erlenmeyer flask withground stopper was placed on a hot plate provided with a magneticstirrer, and stirring was made for 30 minutes while the toluene wasrefluxed, to completely dissolve the sizing agent polymer adhered to thecarbon fiber of strand.(4) The carbon fiber was taken out from the toluene solution. Theoperations of (2) and (3) were repeated two more times with freshtoluene.(5) The toluene solutions containing the sizing agent polymer weretransferred into a rotary evaporator and the toluene was vaporized,followed by measurement of the mass (W2) of the residue obtained.(6) The dry mass (W3) of the sizing agent-extracted carbon fiber wasmeasured.

The amount of adhered sizing agent and the amount of adhered polyolefinresin were determined from the following formulas (i) and (ii).

Amount of adhered sizing agent=(W1−W3)×100  (i)

Amount of adhered polyolefin resin=W2/W3×100  (ii)

[Method for Measurement of Amount of Carbon Fiber's Fluff Generated byFretting]

Five, chromium-plated, stainless steel round rods 3 of 2 mm in diameterwere arranged zigzag as shown in FIG. 1. They were arranged so that thedistance A between adjacent stainless steel round rods 3 was 15 mm andthe bending angle α of sizing agent-adhered carbon fiber strand 1 was120°. A sizing agent-adhered carbon fiber strand 1 was withdrawn from abobbin and stretched zigzag along the stainless steel round rods. Therelease tension of the carbon fiber strand withdrawn from the bobbin wasset at 1.96 N (200 gf) and the carbon fiber strand was subjected tofretting by the stainless steel round rods.

The carbon fiber strand after fretting was interposed between twourethane sponge sheets (dimension: 32 mm×64 mm×10 mm, mass: 0.25 g), anda weight of 125 g was applied on the whole surface of the urethanesponge sheets. Under this conduction, the carbon fiber strand was passedthrough the urethane sponge sheets for 2 minutes at a speed of 15 m/min.Then, the amount of the fluff adhered to the sponge was measured andtaken as the amount of fluff generated by fretting.

[Measurement of Drape]

Drape was measured under the following conditions, using HydrometerModel HOM-2 (a product of Daiei Kagaku Seiki Seisakusho).

Sample for measurement: a carbon fiber strand of 20 cm in length

Slit width: 10 mm

Method for measurement: As shown in FIG. 2, a sample 20 was mounted on asample stand 24 at an angle normal to the slit of the sample stand. Theposition of the sample 20 was adjusted so that the center of the sample20 was above the slit 2. Then, the sample was forced into the slitperpendicularly to a depth of 10 mm at a rate of 10 mm/sec, using ametal plate 26 of 2 mm in thickness and 200 mm in length. The maximumload applied to the metal plate 26 was measured using a load cell (notshown) fitted to the metal plate 26. The measurement was conducted fivetimes using different samples and its average value was taken asmeasured value (F).

Here, drape is defined by the following formula (iii).

Drape [MPa]=F/(S/10)  (iii)

wherein F is a measured value [gf] given by the Hydrometer and S is asectional area of carbon fiber strand [mm²].

The sectional area S of carbon fiber strand was calculated by sectionalarea (s) of carbon fiber filament×number of filaments.

[Winding Density of Carbon Fiber Strand Package]

A carbon fiber strand was wound round a bobbin to form a package. Usinga calipers, there were measured the winding diameter (d1) and windinglength (traverse width) (L) of package and the diameter (d2) of bobbin.Next, using a balance, there were measured package mass (W4) and bobbinmass (W5). From these measurements, winding density was calculated usingthe following formula (Iv).

Winding density=(W4−W5)/{π/4×(d1² −d2)² ×L}  (iv)

[Evaluation of Adhesion Strength Between Carbon Fiber Filament andPolypropylene Resin]

Adhesion strength was evaluated by a micro-droplet method, using atester for interfacial property of composite material, produced by ToeiSangyo Co., Ltd.

The evaluation method used is as follows. First, a carbon fiber filamentwas taken out from each of the carbon fiber strands produced in Examplesand Comparative Examples. As shown in FIG. 3, the two ends of the carbonfiber filament 31 were fixed to the projected portions 33 a and 33 b atthe two ends of a U-shaped backing paper 33, using an adhesive 32. Bythis operation, the carbon fiber filament 31 was stretched tightlybetween the projected portions 33 a and 33 b of backing paper 33. Thebacking paper was fixed to the backing paper holder of a tester. Thebacking paper holder was connected to a sample-moving apparatus providedwith a load cell, in such a way that the backing paper 33 could be movedat a given speed to the fiber axis direction of the carbon fiberfilament 31.

A polypropylene resin (J-900 GP, a product of Idemitsu petrochemicalCompany, Limited) heated at 200° C. and melted was suspended in the formof liquid droplets from the meshes of a sample container provided in thetester and was allowed to contact with the carbon fiber filament 31stretched with the backing paper 33. By this operation, there wasobtained a sample for measurement comprising the carbon fiber filament31 and micro-droplets 34 adhered thereto. The micro-droplets 34 werecooled sufficiently at room temperature, after which the carbon fiberfilament 31 was held by two SUS-made blades 35 a and 35 b. Then, thebacking paper 33 was moved to the fiber axis direction of the carbonfiber filament 31 at a speed of 0.06 mm/min, whereby the carbon fiberfilament 31 was pulled out from the micro-droplets 34 and the maximumload F at pulling-out was measured by the load cell. The measurement wasmade at an atmosphere temperature of 23° C. in a nitrogen atmosphere.

Interfacial shear strength τ was calculated from the following formula(v) to evaluate the adhesion strength between carbon fiber filament andpolypropylene resin.

τ=F/πdl  (v)

In the formula (iii), F is the maximum load at pulling-out, d is thediameter of carbon fiber filament, and 1 is the particle diameter ofmicro-droplets in the pulling-out direction.

[Measurement of Crystallinity by IR Spectrometry]

Using a freeze-grinder (Model JFC 300, a product of Yoshida Seisakusho),a resin to be used as a sizing agent was cooled to −50° C. and made intoa fine powder. Then, the resin powder and a KBr powder were groundsufficiently in a mortar and mixed uniformly. Thereafter, the mixturewas molded into a disc shape using a tablet molder to produce a sample.The thus-obtained sample was measured for IR spectra at an atmospheretemperature of 23° C. in a nitrogen atmosphere, using FT-IR (Magna-IR™spectrometer 550, a product of Nicolet). The density d (g/cm³) of theresin used was determined from the ratio (A₉₉₈/A₉₇₃) of the absorbanceA₉₉₈ of 998 cm⁻¹ and the absorbance A₉₇₃ of 973 cm⁻¹, using thefollowing formula (Iv), and the crystallinity X of the resin wascalculated from the following formula.

Density d=(A ₉₉₈ /A ₉₇₃+9.513)/11.44

Crystallinity X(%)=(0.936/d)×(d−0.850)/(0.936−0.850)×100

Examples 1 to 16 and Comparative Examples 1 to 7 (1) Preparation ofAcid-Modified Polyolefin Copolymer Resin (Component A) Suspension

100 g of a main chain [ethylene-propylene copolymer (E-P),propylene-butene copolymer (P-B) or ethylene-propylene-butene copolymer(E-P-B)] shown in Table 1 was mixed with 400 g of toluene. The mixturewas heated with stirring, in an autoclave, to obtain a solution. Whilethe autoclave-inside temperature was kept at a temperature at leastequal to the melting point of the polyolefin copolymer resin, there wereadded maleic anhydride, methyl methacrylate and perbutyl I to graftmaleic anhydride and methyl methacrylate onto the polyolefin copolymer.

Next, to 25 g of the acid-modified polyolefin copolymer resin obtainedwere added 100 g of water, a polyoxyethylene alkyl type surfactant,potassium hydroxide and morpholine. The mixture was heated with stirringto completely melt the acid-modified polyolefin copolymer to obtain asuspension of the acid-modified polyolefin copolymer resin. However, itwas impossible to convert the acid-modified polyolefin resin of lowgrafting degree, of Comparative Example 6 into a suspension.

(2) Preparation of Acid-Modified Polypropylene Resin (Component B)Suspension

100 g of a polypropylene resin was mixed with 400 g of toluene. Themixture was heated with stirring, in an autoclave, to obtain a solution.While the autoclave-inside temperature was kept at a temperature atleast equal to the melting point of the polypropylene resin, there wereadded maleic anhydride, methyl methacrylate and perbutyl I to graftmaleic anhydride and methyl methacrylate onto the polypropylene.

Next, to 25 g of the acid-modified polypropylene resin obtained wereadded 100 g of water, a polyoxyethylene alkyl type surfactant, potassiumhydroxide and morpholine. The mixture was heated with stirring tocompletely melt the acid-modified polypropylene resin to obtain asuspension of the acid-modified polypropylene resin. However, it wasimpossible to convert the acid-modified polypropylene resin ofComparative Example 7 (the amount of unsaturated carboxylic acid graftedwas small) and the acid-modified polypropylene resin of ComparativeExample 3 (the molecular weight of polypropylene was large) intorespective suspensions.

(3) Production of Carbon Fiber Strand

There were mixed, at a given mixing ratio, the modified polyolefincopolymer resin suspension prepared in (1) and the modifiedpolypropylene resin suspension prepared in (2). The resulting suspensionmixture was diluted with pure water so that the concentration of thetotal resins [the component (A) plus the component (B)] became 25g/liter.

In the diluted suspension mixture was continuously immersed a carbonfiber strand containing no sizing agent, to impregnate the sizing agentbetween the filaments of strand. Successively, the strand was passedthrough a dryer (atmosphere temperature: 140° C.) for 3 minutes toevaporate the water contained in the strand, to obtain a sizingagent-adhered carbon fiber strand. Incidentally, the maximum temperatureof the carbon fiber strand in the dryer was 120° C.

Then, the carbon fiber strand was heated up to 200° C. using a farinfrared heater to obtain a carbon fiber strand of the presentinvention.

The used carbon fiber strand containing no sizing agent was BesfightSTS-24K N00 produced by Toho Tenax Co., Ltd. [diameter 7 μm×24000filaments, fineness: 1.6 g/m, tensile strength: 4000 MPa (408 kgf/mm²),tensile modulus: 238 Gpa (24.3 ton/mm²)].

The obtained carbon fiber strand was measured for amount of resincomposition adhered, the mass ratio of component A and component B inadhered resin, adhesion strength to polypropylene resin and amount offluff generated by fretting. The results are shown in Tables 1 and 2,together with the amounts of component A and component B grafted and themolecular weights of component A and component B.

TABLE 1 Resin composition Amount of Component A Component B Amount offluff Kind of Amount of Amount of adhesion Mass Adhesion generated maingraft Molecular graft Molecular (mass ratio strength by fretting chain(mass %) weight (mass %) weight parts) A/B (MPa) (mg) Example 1 E-P 558000 5 28000 1 3/7 11.7 9.2 Example 2 E-P 5 58000 5 28000 1 10/5  10.67.1 Example 3 E-P 5 58000 5 28000 1  1/10 13.1 10.3 Example 4 E-P 363000 5 28000 1 3/7 11.1 9.0 Example 5 E-P 10 60000 5 28000 1 3/7 11.99.3 Example 6 E-P 5 58000 3 30000 1 3/7 10.7 9.5 Example 7 E-P 5 58000 818000 1 3/7 11.9 8.9 Example 8 E-P 5 58000 15 37000 1 3/7 11.7 9.1Example 9 E-P 5 20000 5 28000 1 3/7 11.1 9.9 Example 10 E-P 5 120000 528000 1 3/7 11.4 6.8 Example 11 E-P 5 58000 5 4500 1 3/7 11.1 10.0Example 12 E-P 5 58000 5 50000 1 3/7 11.6 9.2 Example 13 E-P-B 5 62000 528000 1 3/7 10.9 9.0 Example 14 E-P 5 58000 5 28000 0.5 3/7 11.1 10.6Example 15 E-P 5 58000 5 28000 3 3/7 11.9 9.0 Example 16 E-P 5 58000 528000 6 3/7 12.1 10.3

TABLE 2 Resin composition Amount of Component A Component B Amount offluff Kind of Amount of Amount of adhesion Mass Adhesion Generated maingraft Molecular graft Molecular (mass ratio strength by fretting chain(mass %) weight (mass %) weight parts) A/B (MPa) (mg) Remarks Comp. E-P5 58000 — — 1 10/0  8.0 6.3 Example 1 Comp. — — — 5 28000 1  0/10 13.315.2 Example 2 Comp. E-P 5 58000 5 150000 — — — — Component B Example 3was impossible to emulsify. Comp. E-P 5 58000 5 28000 0.08 3/7 9.7 14.7Example 4 Comp. E-P 5 58000 5 28000 8.5 3/7 12.0 10.9 Winding Example 5density was low. Comp. E-P 0.1 58000 5 28000 — 3/7 — — Component AExample 6 was impossible to emulsify. Comp. E-P 5 58000 0.1 35000 — 3/7— — Component B Example 7 was impossible to emulsify.

The carbon fiber strands obtained in Example 2 and Comparative Example 5were measured for drape and winding density of carbon fiber strandpackage. The results are shown in Table 3.

TABLE 3 Drape (MPa) Winding density Example 1 49 1.02 Comparative 630.80 Example 5

Example 17

Using the component [A] (melting point: 70° C.) and the component [B](melting point: 155° C.) both used in Example 1, respective suspensionswere produced. The obtained suspension of the component [A] wasimpregnated into a carbon fiber strand, followed by drying. Then, thesuspension of the component [B] was adhered to the resulting carbonfiber strand, followed by drying. Thereafter, the carbon fiber strandwas heated up to 200° C. using a far infrared heater to obtain a carbonfiber strand of the present invention. The amounts of the component [A]and the component [B] adhered, etc. were the same as in Example 1.

Using this carbon fiber strand, the same evaluation tests as in Example1 were conducted. As a result, the adhesion strength was 11.9 MPa andthe amount of fluff generated by fretting was 8.9 mg.

Example 18

Using the component [A] (melting point: 70° C.) and the component [B](melting point: 155° C.) both used in Example 1, respective suspensionswere produced. The obtained suspension of the component [B] wasimpregnated into a carbon fiber strand, followed by drying. Theresulting carbon fiber strand was heated up to 200° C. (this temperaturewas at least equal to the melting point of the component [B]) and keptat that temperature for 30 seconds. Then, the suspension of thecomponent [A] was adhered to the carbon fiber strand, followed bydrying. Thereafter, the carbon fiber strand was heated at 120° C. for 30seconds to obtain a carbon fiber strand of the present invention. Theamounts of the component [A] and the component [B] adhered, etc. werethe same as in Example.

Using this carbon fiber strand, the same evaluation tests as in Example1 were conducted. As a result, the adhesion strength was 12.0 MPa andthe amount of fluff generated by fretting was 8.8 mg.

Examples 19 to 24 Production Example of Component [B]

There were mixed 100 g of a raw material polypropylene having amolecular weight of 15,000 to 300,000 and 400 g of toluene. The mixturewas heated with stirring, in an autoclave, to obtain a solution. Theretowas dropwise added an organic peroxide (dicumyl peroxide) with theautoclave-inside temperature kept at a temperature at least equal to themelting point of the resin. The resulting mixture was subjected to athermal degradation treatment. Next, there were added an unsaturatedcarboxylic acid (maleic anhydride) and an organic peroxide (perbutyl) tograft the unsaturated carboxylic acid to the polypropylene. After thecompletion of the reaction, the reaction product was poured into a largeamount of methyl ethyl ketone for purification.

Next, 25 g of the modified polypropylene resin obtained was uniformlydissolved in a solvent (toluene). There were placed, in anemulsification vessel, the toluene solution of the modifiedpolypropylene resin and a separately prepared, aqueous surfactantsolution, followed by stirring, to obtain a pre-emulsion.

Diethanolamine was added to the pre-emulsion for pH adjustment. ThepH-adjusted pre-emulsion was subjected to vacuum distillation forremoval of toluene, using a rotary evaporator to finally obtain asuspension of a modified polypropylene resin.

In the suspension was continuously immersed a carbon fiber strandcontaining no sizing agent (STS-24K N00, a product of Toho Tenax Co.,Ltd.), to impregnate the suspension between the filaments. Successively,the suspension-impregnated strand was passed through a dryer (atmospheretemperature: 140° C.) for 3 minutes, to evaporate the water to obtain amodified polypropylene resin-adhered, carbon fiber strand. Incidentally,the maximum temperature of the carbon fiber strand in the dryer was 120°C.

The acid value, crystallinity, intrinsic viscosity and mass reduction ofthe resin adhered to (contained in) the obtained carbon fiber strand areshown in Table 4.

TABLE 4 Crystal- Intrinsic Mass Melting Acid value linity viscosityreduction point (mg-KOH/g) (%) (dl/g) (%) (° C.) Example 19 79 61 0.232.6 148 Example 20 53 66 0.17 2.8 150 Example 21 85 59 0.15 3.4 155Example 22 75 68 0.14 3.2 156 Example 23 48 74 0.24 2.3 153 Example 2459 74 0.65 2.0 155

Then, to each of the carbon fiber strands of Examples 19 to 24containing the above sizing agent of component [B] was adhered thesizing agent (melting point: 70° C.) of the component [A] used inExample 17, followed by drying. Then, the resulting carbon fiber strandwas heated at 200° C. for 1 minute, to obtain a carbon fiber strand ofthe present invention.

Examples 25 to 28 and Comparative Examples 8 to 9

The component [A] (melting point: 70° C.) and the component [B] (meltingpoint: 155° C.), both used in Example 1 were mixed at a mass ratio of1/9 to produce a suspension. The suspension was impregnated into acarbon fiber strand, followed by drying. The resulting carbon fiberstrand was heated at 120 to 220° C. as shown in Table 5, to obtain acarbon fiber strand of the present invention. The adhered amount of thecomponent [A] and the component [B] was 1%.

Using this carbon fiber strand, the same fluff amount generated byfretting, as in Example 1 was measured. The results are shown in Table5.

TABLE 5 Fluff amount Temperature of generated by Example heating (° C.)fretting (mg) Comparative 120 32.4 Example 8 Comparative 140 28.0Example 9 Example 25 160 11.1 Example 26 180 10.2 Example 27 200 9.1Example 28 220 10.5

1. A carbon fiber strand for reinforcement of thermoplastic resin,wherein a sizing agent containing a resin composition obtained by mixingthe following components [A] and [B]: [A] an acid-modified polyolefincopolymer having a weight-average molecular weight of 15,000 to 150,000,which has, as the main chain, at least one member selected from anethylene-propylene copolymer, a propylene-butene copolymer and anethylene-propylene-butene copolymer and in which the main chain has beenmodified with 0.1 to 20% by mass of an unsaturated carboxylic acid, and[B] an acid-modified polypropylene having a weight-average molecularweight of 3,000 to 150,000, which has, as the main chain, apolypropylene and in which the main chain has been modified with 0.1 to20% by mass of an unsaturated carboxylic acid, at a mass ratio of 1:20to 10:5, is adhered to 100 parts by mass of a carbon fiber in an amountof 0.1 to 8.0 parts by mass.
 2. A carbon fiber-reinforced thermoplasticresin obtained by adding, to a thermoplastic resin, a carbon fiberstrand for reinforcement of thermoplastic resin, set forth in claim 1,in an amount of 5 to 70% by mass.
 3. The carbon fiber-reinforcedthermoplastic resin according to claim 2, wherein the thermoplasticresin is a polypropylene.
 4. A method for producing a carbon fiberstrand for reinforcement of thermoplastic resin, which comprisesimmersing a carbon fiber strand in an aqueous suspension containing aresin composition set forth in claim 1 and then heating the resultingcarbon fiber strand at a temperature at least equal to the melting pointof the resin composition.
 5. A method for producing a carbon fiberstrand for reinforcement of thermoplastic resin, which comprisesimmersing a carbon fiber strand in either one of an aqueous suspensionof [A] an acid-modified polyolefin copolymer and an aqueous suspensionof [B] an acid-modified polypropylene, drying the resulting carbon fiberstrand, immersing the dried carbon fiber strand in the other aqueoussuspension, and then heating the carbon fiber strand after immersion inthe two aqueous suspensions, at a temperature at least equal to themelting point of the resin composition.
 6. A method for producing acarbon fiber strand for reinforcement of thermoplastic resin, whichcomprises immersing a carbon fiber strand in an aqueous suspension of[B] an acid-modified polypropylene, drying the resulting carbon fiberstrand, heating the dried carbon fiber strand at a temperature at leastequal to the melting point of the acid-modified polypropylene, immersingthe resulting carbon fiber strand in an aqueous suspension of [A] anacid-modified polyolefin copolymer, drying the resulting carbon fiberstrand, and then heating the dried carbon fiber strand at a temperaturewhich is at least equal to the melting point of the acid-modifiedpolyolefin copolymer but lower than the melting point of theacid-modified polypropylene [B].
 7. A method for producing a carbonfiber strand for reinforcement of thermoplastic resin, set forth inclaim 4, wherein the acid-modified polypropylene [B] has the followingproperties: (1) a crystallinity of 50 to 80% as measured by IRspectrometry, (2) an intrinsic viscosity [η] of 0.05 to 0.7 dl/g, (3) anacid value of 40 to 100 mg-KOH/g, and (4) a mass reduction of less than5% when subjected, by a thermobalance, to temperature elevation from 23°C. to 250° C. at a rate of 10° C./min in the air.
 8. A method forproducing a carbon fiber strand for reinforcement of thermoplasticresin, set forth in claim 5, wherein the acid-modified polypropylene [B]has the following properties: (1) a crystallinity of 50 to 80% asmeasured by IR spectrometry, (2) an intrinsic viscosity [η] of 0.05 to0.7 dl/g, (3) an acid value of 40 to 100 mg-KOH/g, and (4) a massreduction of less than 5% when subjected, by a thermobalance, totemperature elevation from 23° C. to 250° C. at a rate of 10° C./min inthe air.
 9. A method for producing a carbon fiber strand forreinforcement of thermoplastic resin, set forth in claim 6, wherein theacid-modified polypropylene [B] has the following properties: (1) acrystallinity of 50 to 80% as measured by IR spectrometry, (2) anintrinsic viscosity [η] of 0.05 to 0.7 dl/g, (3) an acid value of 40 to100 mg-KOH/g, and (4) a mass reduction of less than 5% when subjected,by a thermobalance, to temperature elevation from 23° C. to 250° C. at arate of 10° C./min in the air.